Edit css for search results page

I bet that you’re misunderstood what is_page_template is really doing. From Codex:

This template tag allows you to determine if you are in a page
template. You can optionally provide a template name or array of
template names and then the check will be specific to that template.

And if you’ll take a look at it’s code:

function is_page_template( $template="" ) {
    if ( ! is_singular() ) {
        return false;
    }
 
    $page_template = get_page_template_slug( get_queried_object_id() );
    ...

… you’ll get, that you can’t use this function to target search results. It won’t work, because it’s meant to check if current page uses given page template. On the other hand you want to check if given file is currently used…

If your theme is coded correctly, then you should use is_search() instead.